1. Field of the Invention
The present invention relates to a position-measuring device for detecting the relative position of two bodies capable of motion relative to each other, the device comprising a measurement standard with a scale division connected to one of the bodies and a scanning head connected to the other body for reading the scale division, wherein the scanning head is connected to an electronic evaluation unit.
2. Description of the Related Art
Measuring devices with multiple scanning for accurate detection of angles are known in the prior art. An angle-measuring device of this type consists basically of a measurement standard in the form of a disk-shaped carrier with a measurement scale division and one or more scanning units. When the two elements move relative to each other, the scanning unit detects the angular position of the measurement standard in absolute or incremental form. It is extremely important that such devices measure the angle with extremely high accuracy. This accuracy is influenced essentially by the following system properties:                the eccentricity of the overall measurement scale division with respect to the rotational axis;        the deviations of the shape of the scale division carrier from the ideal cylindrical shape;        division errors in the form of local geometric deviations from the ideal form of the coding in the rotational direction or perpendicular to it;        the runout of the bearings;        aleatoric geometric deviations such as bearing play or material deformation under load; and        deviations in the detection of the angular position caused by the electronic evaluation unit.        
It is a general principle of the scanning of periodic scale divisions that it is not sufficient to use a single scanning head with a sensor unit to scan a single period (division mark) of the scale division. Instead, several such periods are detected simultaneously not only to generate the desired signals but also to allow the signals to be averaged, so that the individual division mark errors can be suppressed. This means that, by electronically calculating the average value of the signals generated by the individual division marks, a more accurate set of angle data can be obtained for the scanned range of the scale division. This principle extrapolates; that is, a complete circular measurement around the entire circumference of the measurement standard should yield ideal results. As will be explained below, this type of scanning also suppresses all of the other sources of error mentioned above. This system design can seldom be technically realized in a practical manner, and for this reason various substitute solutions have been developed over the course of time.
Most of these solutions are based on double diametric scanning, which completely suppresses the errors caused by the eccentricity of the scale division and reduces the remainder of the other deviations to a mean value. An exact explanation of this principle can be found in the following description.
An important aspect of the summation of the individual signals from the two scanning units is that they must be in phase with only a small allowable deviation (phase difference approaching “0”) to prevent any reduction in the amplitude. Otherwise, in the extreme case, i.e., in the case of a phase difference of 180° (counterphase), the overall signal would be reduced to a sum of “zero”.
DE 1 811 961 discloses an angle-measuring device with diametrically opposed scanning units. Four scanning units are used for error compensation. To solve the phase difference problem, a second, round reference scale division with its own diametrically opposing scanning units is used in addition to the primary measuring scale division of the measurement standard. This solution is highly complex and is expensive to implement.
EP 0 302 194 B1 describes a circuit for monitoring the phase difference between individual scanning units of an angle-measuring device with diametrically opposed detection sensors and for suppressing the possible error which can occur when the phase difference between the individual scanning units is exceeded. This prevents an irreversible loss of the angle values, and the division periods which have already been completely scanned are preserved.
DE 19 920 596 C2 describes a special complex electronic circuit for converting all the individual analog input signals from the various diametrically opposing scanning units of an angle-measuring device into digital values. It also describes how these signals are compensated to obtain the measurement data.